Five distinct measures of population fitness will be compared using strains of Drosophila melanogaster that differ in their overall fitness. The methods are (1) the compound-autosome method of Jungen and Hartl, (2) the method of Knight and Robertson, (3) the method of Sved and Ayala, (4) equilibrium biomass, and (5) interspecific competitive ability. At issue is whether these methods are highly correlated, which would suggest that they measure different aspects of the same thing, or whether there is little relationship among them. In addition, the power of the compound-autosome method to detect differences in overall fitness will be assessed using computer simulation and studies of artificial and natural populations. Although the field of population genetics is suffused with the concept of fitness, the term is used in various ways, sometimes alluding to intraspecific competitive ability (methods 1 to 3 above), sometimes to population size or biomass (method 4), and still other times to interspecific competitive ability (method 5). The significance of this research is that it will determine empirically whether these different concepts are operationally equivalent or nearly equivalent. If the compound-autosome technique proves sufficiently powerful to detect differences in fitness of the magnitude thought to occur in natural populations, then the method will be used to investigate predictions of the shifting balance theory of evolution. This so-far untested theory has wide-ranging implications for understanding the population structure and evolution of virtually all sexual eukaryotes, in particular that of human beings.